1. Field of the Invention
The present invention relates to a dynodes arrangement for an electron multiplier. In particular, the invention is utilized in scintillation gamma cameras for nuclear diagnosis purposes.
2. Description of the Prior Art
It is known that the gain of photomultiplier tubes (PMT's), in particular those which are utilized in scintillation gamma cameras, changes with time. Due to this, PMT's have to be retuned from time to time.
The British Patent Application No. 2,067,281 describes a method and circuitry for retuning by means of an automatic gain control (AGC) amplifier.
Further, the British Patent No. 977,827 illustrates a method and circuitry for retuning by modifying the potential of the anode and some of the dynodes, when the total gain of the photomultiplier exceeds a predetermined value, by directly connecting together the anode and a number of dynodes which are closest to the anode.
Finally, the European Patent Application No. 0,066,763 delineates a method and circuitry for retuning by means of varying the potential difference between to neighboring dynodes.
However, it is also possible to retune the gain of a photomultiplier by means of varying the of high voltage.
The aforementioned two methods, namely controlling gain in a photomultiplier tube for retuning by means of varying the interdynode potential differences or by means of changing the high voltage are the easiest methods. However, these methods have also some limitations. These limitations include for example
(a) that the peak linear and peak saturated output current from the photomultiplier tube is reduced in the reduced gain state; PA1 (b) that non-linear effects occur as the gain is decreased; PA1 (c) that the transit time of the photomultiplier tube is increased as the gain is decreased; and PA1 (d) that the gain changes with the count rate (e.g., increased count-rate-shift effect at decreased gain).
Page 3 of the brochure "Nucleonics Data", issued by Johnston Laboratories, Cockeysville, Md. 21030, Number JLI-605, illustrates a dynodes arrangement wherein a grid is positioned between each two neighboring dynodes. The grid is always electrically connected with one of the two neighboring dynodes.
The copending application Ser. No. 343,207, filed Jan. 27, 1982, by Dennis E. Persyk, entitled "Radiation Detector Assembly for Generating a Two-Dimensional Image" shows a radiation detector having a photocathode, an electron multiplier such as a multichannel plate, and a grid positioned between the photocathode and the electron multiplier. A first electrical field is provided between the photocathode and the grid and a second electrical field is provided between the grid and the electron multiplier; whereby the strength of the second electrical field is larger than the strength of the first electrical field. Due to this the impingement area of a packet of photo electrons on the electron multiplier input is enlarged and thus, simultaneously, the electron density is reduced, which in certain applications is a favorable result.